Method of digesting wood by the kraft process utilizing organic sulfides



June 24, 1969 n o ET AL 3,451,889

METHOD OF DIGESTING woon BY THE KRAFT PROCESS UTILIZING ORGANIC SULFIDES I Filed Aug. 28, 1964 Sheet of 5 Fl 6.1 27 v 2% lnerl' Pesi'cluui f A Gases GAS CONDENSER OR ABSORBER Candcnnd Turpelfline and Wahr paqocqq J0 pzsuapuo an/m D\GESTER apunodmo mans Addil'iona 50d? compwnb if desirable A. TIRADO ET AL June 24, 1969 METHOD OF DIGESTING WOOD BY THE KRAFT PROCESS UTILIZING ORGANIC SULFIDES Sheet Filed Aug. 28, 1964 BLEACHED UP TO 80' GE BRIGHTNESS 2 W v R O 6 6 N 6 m m 7 D M m m J R T W. W m R w O.\ 0\ F.3 0 6 N 0 5 2 U V. 8 R A L D T T M m f R T a m N R I T W O.\ a F 3 o o T O z L .Y I O A R D M H O T c a K G W 8 6 4 T 8 6 4 2 w w w w w w m w m wzu\ uz zw 5z 2 a l v 8 4 2 W0 d0 HEOQZUZJM mQJOm M 525 55 026 6. N w 0 INVENTORS ADALBERTO TIRADO BEATING TIME MINUTES ATTORNEY.

June 24, 1969 A. TIRADO ET AL 3,451,889

METHOD OF DIGESTING WOOD BY THE KRAFT PROCESS UTILIZING ORGANIC SULFIDES Filed Aug. 28, 1964 Sheet 4 of 5 FIG. 4.

CONTROL TEST- CONVENTIONAL KRAFT COOKS WITHOUT NEW SULFUR COMPOUNDS 9,470 460 O 439 5' ,450 2 m o 43D til E 420 AVERAGE CONVENTIONAL SULFIDITY=23.O4% UJ 59% 5,&JC5\ 4| 515 1468|||3|5|820222426282579|2|4|s 35 2579l2|4l6l92l232527l468l||3|5|7 35 HFEBRUARYQH |1--MARCHl-+|l963 FIRST TRIAL. NEW SULFUR COMPOUNDS BEING ADDED AVERAGE SEEMING SULFIDITY 22.95

KG. NQ O/METRIC TON OF A.D. PULP (:1

ACCUMULATIVE AVERAGE OF ACTIVE ALKALI USED IN DIGEST R5 in 4 6 8 IO 19. 24 302468|O 24 28303579 I963 |'MARCH- 1 kAPRu --+4 ld--MAY-fi INVENTORS ADALBERTO TIRADO MARIO GUEVARA BY VIRGILIO GONZALES- ATTORNEY.

mo SECOND TRIAL- NEW SULFUR COMPOUNDS BEING ADDED mn o: 91460 I I 3 2 A 4 v g; g Q AVERIIXGE ssgzwm uc, SUL'FIDIITYF 2 1.l9

United States Patent 3,451,889 METHOD OF DIGESTING WOOD BY THE KRAFT PROCESS UTILIZING ORGANIC SULFIDES Adalberto Tirado, Mario Guevara, and Virgilio Gonzalez,

T lalpan, Mexico, assignors to Fabricas de Papel Loreto y Pena Pobre 8A., Villa Obregon, Mexico, a corporation of Mexico Filed Aug. 28, 1964, Ser. No. 393,495 Int. Cl. D21c 3/20 US. Cl. 162-72 6 Claims ABSTRACT OF THE DISCLOSURE Wood pulp is produced by the kraft process operated by utilizing an alkaline cooking liquor containing not more than about 90% sodium sulfide, the balance of the sulfidity of the liquor being furnished by organic sulfurcontaining compounds containing two sulfur-connected valences and comprising either pure mercaptans or organic sulfides or disulfides or compounds recovered from the gases relieved from the digesters.

This invention relates to improvements in the production of wood pulp by the so-called kraft process.

More partocularly, the present invention is concerned with improvements in the production of wood pulp by the aforementioned process and which provides a number of highly desirable advantages over the heretofore known kraft processes for production of wood pulp.

In the heretofore known kraft processes as practised in pulp and paper mills for the production of wood pulp, inorganic sulfides, particularly sodium sulfide, constitute active chemicals in the cooking of the wood. These inorganic sulfides are utilized because they are necessary in order to obtain pulp possessing several desirable characteristics, particularly those pertaining to the strength and other properties of the bleached or unbleached pulp and of the finished paper produced from the pulp.

The principal object of the invention is to provide a method for production of wood pulp by the kraft process,

utilizing inorganic sulfides such as more particularly sodium sulfide, but wherein there will be obtained an increase in the yield of pulp for a given sulphidity of the cooking liquor.

Another object of the invention is to provide a process as aforesaid and wherein the amount of alkali consumed in the cooking process may be substantially reduced.

As will be readily apparent to those skilled in the art, the achievement of either of the foregoing objects, and the more so of both, will be of highly desirable commercial advantage, especially considering the economic factors entailed in the operation of a kraft mill.

Still another object of the invention is to provide a process as aforesaid and wherein at the same time the properties of the pulp, such as its strength and bleachability, are not adversely affected, and indeed may be substantially improved, notwithstanding the increase in the yield of the pulp and/or the decrease in the amount of alkali consumed in the process.

Other objects and advantages of the invention will be apparent from the detailed description herebelow:

In the accompanying drawing:

FIG. 1 illustrates, in the form of a How diagram, an embodiment of the invention as it may preferably be practised in a kraft mill;

FIG. 2 is a chart composed of graphs showing the results of various strength tests of paper made from the pulp obtained in the Control Test Run and in Trial Runs made according to the invention; and

FIGS. 3, 4 and 5 are graphs showing, respectively,

(a) the accumulated average yield, (b) the accumulated average of active alkali consumed, and (c) the accumulated average of permanganate number, obtained in the Control Test Run and in each of the two Trial Runs more fully described herebelow.

Broadly .stated, the objects of the invention are achieved by partially or totally substituting for the sodium sulfide conventionally used in the kraft process, certain organic sulfide-containing compounds, which when added to the alkaline liquor, render the alkali more active toward the lignin and allied constituents of the Wood.

More particularly, in pursuance of the foregoing, the organic sulfur-containing compounds utilized in the practice of the invention are those which contain the sulfur in bivalent form, i.e., compounds which contain two sulfurconnected valences, as is the case in mercaptans and other organic sulfides and disulfides.

It has been found, in accordance with the invention, that when all or a substantial portion of the sulphidity of the alkaline cooking liquor consists of the above-mentioned organic sulfur-containing compounds, instead of consisting entirely of or at any rate more than about ninety percet of sodium or other inorganic sulfide, the yield of pulp increases while, at the same time the quantity of active alkali required decreases, the permanganate number of the pulp remains the same, and the strength and other properties of the pulp, particularly those which affect the folding strength of the paper made from the pulp, are likewise the same or even somewhat improved.

Accordingly, the sulfidity of the alkaline cooking liquor consists of not more than about sodium sulfide, the remainder of the sulfidity consisting of the herein specified organic sulfur-containing compounds.

Thus, it has been found that if, for example, 25% total sulphidity is used, and this total sulphidity is composed of two fractions, namely, 22% actual sodium sulfide and 3% seeming or apparent sodium sulfide, the latter fraction consisting of one or a mixture of the above-mentioned organic sulfur-containing compounds, such as mercaptans, organic sulfides and disulfides, there results an increase of the order of 2.0 to 2.5% in the yield of pulp, accompanied by a decrease of the order of 2 to 3% in the active alkali requirement, as compared, respectively, with the yield and active alkali requirement in the case of pulp obtained when operating under the same conditions except that the 25% total sulphidity is derived entirely from actual sodium sulfide. In the foregoing, as well as in the description herebelow, the true as well as the seeming or apparent sulphidity were in all instances determined as equivalent sodium sulfide by titration with iodine, and expressed as Na O.

The organic sulfur-containing compounds which may be utilized in the practise of the invention are those which, as above stated, contain the sulfur in bivalent form and which have a structural formula selected from the group consisting of:

(d) suitable mixtures of two or all of the foregoing quantities of the several components from time to time during a particular run. Thus, in accordance with the invention, advantage may be taken of the organic sulfurcontaining compounds in the form in which they are available in kraft pulp mills, without necessity for purifying or separating the different components. We have found that in commercial scale operation, utilizing these organic sulfur-containing compounds as available in kraft pulp mills, small variations of the gases delivered from the digesters affect the general result only to a minor and substantially insignificant extent.

In order to demonstrate and evaluate the effect of the organic sulfur-containing compounds herein referred to, in the production of wood pulp by the kraft process, the mixed sulfur-containing compounds were collected in a kraft pulp mill where 23% conventional sulfidity, all consisting of actual sodium sulphide, was being used. A control test was run at the same 23% conventional sulfidity, i.e., without the addition of the organic sulfur-containing compounds, for a period of thirty-nine days.

Then a first trial run, extending over a period of fortytwo days, was made under exactly the same conditions as the Control Test Run, except that the 23% sulphidity was composed of two portions or fractions, namely 21.0% actual sodium sulfide and 2.0% seeming sulfidity from organic sulfur-containing compounds. A second trial run of thirty days duration was made, likewise under the same conditions, except that in this instance the sulphidity was increased up to 24.2%, this sulphidity being composed of two portions or fractions, namely, 21.0% actual sodium sulfide and 3.2% seeming sulfidity from organic sulfurcontaining compounds.

In each of the runs, pure Wood was used and the cooking conditions were as follows:

Time to raise pressure in the digesters up to the maximum-25 hours;

Time at maximum constant pressure-0.75 hour;

Maximum pressure-75 kg./cm.

Maximum temperature-175 C.;

Total liquor to wood ratio 1;

Concentration of active alkali40 grams Na O/Iiter.

TABLE 1 Control Test Run Sultidity derived from-- 21% actual NaiS 2% organic sulfur-contaim'ng compounds Trial Run Trial Run 21% actual N a: S 3 2% organic sulfur-containing compounds Duration (days) 39 Average conventional sulfidity, percent 23. 04 Average seeming sulfidity,

percent Accumulative average yield of digestors (metric tons/ cook) Accumulative average of permanganate number (Oestrand Method) Accumulative average of alkali used in digesters (kg. NazO/metric ton of AD. pulp As will be seen from the above tabulation, with an average seeming sulfidity in Trial No. 1 substantially the same as the average conventional sulfidity in the Control Test Run, i.e., approximately 23% in both instances, the

accumulative average yield in the former was 5.26 metric tons per cook, compared to 5.12 metric tons per cook in the latter. The difference of 0.14 metric ton per cook represents an increase of approximately 2.7% yield. At the same time, the accumulative average of alkali used was reduced from 439 to 428 kilograms Na O per metric ton of AD. pulp, i.e., a reduction of approximately 2.5% in Trial Run No. 1 as compared to the Control Test Run. Yet these results were achieved without affect upon the accumulative average of the permanganate number.

With respect to Trial Run No. 2, in which the average seeming sulfidity was increased to 24.19% (as compared to the average conventional sulfidity of approximatey 23 in the Control Test Run), the accumulative average yield was 5.28 metric tons per cook, compared to 5.12 metric tons per cook in the Control Test Run, or an increase of 0.16 metric ton per cook, i.e., an increase of approximately 3.1%. The accumulative average of alkali used was reduced from 439 to 433 kilograms NagO per metric ton of AD. pulp, i.e., a reduction of approximately 1.4%. The accumulative average permanganate number was slightly higher in this trial run than in the Control Test Run (580 compared to 57.25).

As will be noted from the graphs of FIGS. 3 and 4, respectively, the accumulated averages of yield and the accumulated averages of alkali consumption in each of the two Trial Runs reflect a consistent increase therein, as compared to the results obtained in the Control Test Run. These differences, although numerically small, are nevertheless of commercially valuable significance in the practical operation of a kraft mill, as will be apparent to those skilled in the art.

Moreover, as will be seen from the graph of FIG. 5, these advantageous results are obtained in the practise of the invention without substantial effect upon the permanganate number of the resultant pulp.

Data with respect to the resuls of tests on the strengh properies of paper obtained from the pulp produced in the above-mentioned Control and Trial Runs are shown in the graphs of FIG. 2 of the drawing. As there indicated, the data in each instance shows the results at zero heating time and at successive twenty minute intervals of heating time up to a total beating time of the pulp amounting to eighty minutes, and in each case after the pulp was bleached up to GE brightness.

As will be seen from the comparative graphs for the three runs, the results at zero and after eighty minutes of beating time show (a) that the tensile strength has increased from an initial 3.5 to 8.3 in Trial Run No. 1, and from an initial 2.6 to 7.0 in Trial Run No. 2, compared to an increase from an initial 3.2 to 7.6 in the Control Test Run; (b) that the Mullen burst strength increased from an initial figure of approximately 2.0 kg. .per square centimeter in all three cases to figures of 4.0 in Trial Run No. 1 and 3.6 in Trial Run No. 2, compared to a figure of 3.0 in the Control Test Run; (c) that the Elmendorf tear strength went from an initial figure of approximately grams in each of the two Trial Runs to 50 grams in Trial Run No. 1 and 65 grams in Trial Run No. 2, compared to the Control Test Run, in which case it went from an initial 90 grams to 50 grams; and (d) that the folding strength increased from approximately 50 to 260 folds in the case of Trial Run No. 1 and from approximately 20 to 760 folds in the case of Trial Run No. 2, as compared to an increase from approximately 20 to only folds in the case of the Control Test Run.

The utilization of the' organic sulfur-containing compounds, in accordance with the invention, in place of all or a substantial part of the sodium sulfide, will be specially useful and advantageous in those kraft mills which consume only relatively small quantities of sodium sulfide and are accordingly required to add elemental sulfur to the digesters in order to obtain a sufficiently high sulfidity therein.

As hereinabove stated, an important advantage of our invention resides in the fact that the organic sulfur-containing compounds herein referred to are readily available in all kraft mills and may be utilized, without necessity for purifying the same or separating the dilferent components thereof, in practising the method according to the invention. To that end, these compounds may be recovered from the gases which are evolved from the digesters, and recycling the compounds to the digesters. procedure accordingly serves to effect a substantial economy to the mill in improving the yield and reducing the quantity of alkali consumed.

The foregoing procedure is illustrated in FIG. 1 of the drawing. As there shown, the relieved gases from the digester or digesters may be delivered therefrom as overhead gases, through a line 11 to a condenser 12. Condensed components such as turpentine and water are discharged from the condenser through a line 14, while the uncondensed gases are delivered from the top of the condenser through a line 15 and to a gas condenser 16 or absorber 25, from Which the condensed or dissolved organic sulphur compounds are recycled to the digester 10 through lines 26, 17, 18, while the inert residual gases are discharged from condenser 16 or absorber through lines 27, 21, 22. These two alternatives are illustrated in FIG.1, where the condensed or dissolved compounds may be discharged through a line 26 and/or line 17 for recycling the same to the digester. In either event line 18 may be connected with a feed line for feeding additional organic sulphur compounds to the digester, if necessary or desirable.

It is a further advantage of the invention that in the commercial practice thereof, the active alkali in the wood-digesting liquor may be derived either from sodium sulfate plus sodium hydroxide, or from sodium carbonate, which is converted into sodium hydroxide before the digesting process. When employing sodium carbonate, elemental sulfur is preferably also added, in order to obtain the desirable properties of kraft pulp; whereas in the absence of such elemental sulfur, the quality of the pulp resembles the normally lower quality of soda pulp. At any rate, the choice of either sodium sulfate or sodium carbonate as the source of active alkali inthe practice of the invention is of advantage not only from the standpoint of their relative price at any given place and time, but from the additional standpoint that by the practice of the present invention the desirable, or even improved, properties of Kraft pulp are obtained when using sodium carbonate instead of sodium sulfate.

What is claimed is:

1. The method of producing wood pulp which comprises the step of digesting wood in an alkaline cooking liquor containing sulfides as active chemical components of the liquor, the sulfidity of said liquor consisting of not more than about sodium sulfide, the remainder of said sulfidity being an organic sulphur-containing compound available from the papermaking process containing two sulphur-connected valences and having a molecular formula selected from the group consisting of:

(c) RS-SR' (d) a mixture of any two or all three of (a),

in which R is an alkyl radical and R the same as or different from R.

2. The method as defined in claim 1, wherein the total sulfidity of said liquor amounts to from about 23% to about 25% thereof.

3. The method as defined in claim 2, wherein said total sulfidity consists of approximately 21% sodium sulfide and about 2% to about 3% seeming sulfidity consisting of said organic sulfur-containing compound.

4. The method as defined in claim 1, wherein said organic sulfur-containing compound is derived from gases evolved during said digesting of the wood.

5. The method as defined "in claim 2, wherein said organic sulfur-containing compound as derived from gases evolved during said digesting of the wood.

6. The method as defined in claim 3, wherein said organic sulfur-containing compound is derived from gases evolved during said digesting of the wood.

(b), and

is an alkyl radical References Cited UNITED STATES PATENTS 3,313,680 4/1967 Hoar 162-51 X FOREIGN PATENTS 220,779 8/ 1957 Australia.

U.S. Cl. X.R. 162-82 

